This invention relates to a stainless steel having excellent high temperature strength and corrosion resistance and having a scale layer with an excellent ability to shield the steel against carburizing gas. The steel is highly suitable for use in manufacturing a steel pipe or tube (hereafter referred to as “pipe” collectively) capable of being used in a carburizing gas atmosphere containing hydrocarbon gas or CO gas, such as a steel pipe for a cracking furnace, a reforming furnace, a heating furnace, or a heat exchanger employed in a petroleum refinery or a petrochemical plant. The present invention also relates to a stainless steel pipe made from this material.
The present invention also relates to a method of manufacturing a stainless steel having excellent resistance to carburization and coking when used in a carburizing gas atmosphere.
In recent years, due to an increasing demand for synthetic resins, there has been a trend towards the use of higher operating temperatures in cracking furnaces in ethylene manufacturing plants, for example, so as to obtain a higher yield of ethylene. As a result, pipes for use in cracking furnaces are being subjected to higher operating temperatures. The inner surface of pipes used in cracking furnaces are exposed to a carburizing atmosphere at high temperatures, so the pipes need to be made of a heat resistant material having excellent high temperature strength and resistance to carburization.
During operation of a cracking furnace, carbon is deposited on the inner surface of the pipes of the cracking furnace (a phenomenon referred to as coking). As the amount of deposited material increases, operational problems can occur such as an increase in pressure losses (ΔP) and a decrease in heating efficiency. Accordingly, so-called decoking in which the deposited carbon is oxidized and removed using air or steam is periodically carried out. However, it is necessary to stop the operation of a cracking furnace in order to perform decoking, so the operating efficiency of the furnace is greatly decreased by the need to carry out decoking. The problem of coking becomes worse as the diameter of the pipes of a cracking furnace decreases. This is a major drawback, because smaller diameter pipes are advantageous from the standpoint of increasing product yield.
In the past, there have been various proposals of materials for suppressing coking. For example, Japanese Published Unexamined Patent Application Hei 2-8336 proposes a steel pipe which includes at least 28% of Cr so as to form a strong and stable Cr2O3 film on the surface of the pipe to prevent Fe and Ni, which act as catalysts to promote carbon deposition, from floating to the surface of the pipe and to thereby suppress coking.
As disclosed in Japanese Published Unexamined Patent Application Sho 57-23050, for example, it is known that increasing the Si content of an alloy so as to form an SiO2 film on the surface of the alloy is effective at increasing resistance to carburization.
However, in the above-described prior art in which the Cr or Si content of a steel is increased in order to form a film of Cr2O3 or SiO2 on the steel, depending on the operating conditions in an actual carburizing environment, a nonuniform scale layer is formed on the steel surface. If the scale layer undergoes cracking or peeling, it is often not possible for the scale layer to be adequately restored (regenerated).
As a result, the scale layer does not have a satisfactory shielding ability with respect to carburizing gas, so the problem of needing to interrupt equipment operation in order to perform decoking and the problem of deterioration of materials due to carburization remain.
In order to solve these problems of nonuniform formation of scale and inability of a scale to be regenerated, methods have been proposed in which oxidation pretreatment is performed on a steel. For example, Japanese Published Unexamined Patent Applications Sho 53-66832 and Sho 53-66835 disclose a method in which pretreatment of oxidation is carried out on a 25Cr-20Ni (HK 40) low-Si heat resistant steel or a 25Cr-35Ni low-Si heat resistant steel at around 1000° C. in air for at least 100 hours, and Japanese Published Unexamined Patent Application Sho 57-43989 discloses a method in which pretreatment of oxidation in air is carried out on an austenitic heat resistant steel containing 20-35% Cr. In addition, Japanese Published Unexamined Patent Application Hei 11-29776 discloses a method in which resistance to carburization is increased by heating a high Ni—Cr alloy in a vacuum and forming a scale film.
In addition, PCT-based Japanese Published Unexamined Patent Application 2000-509105 discloses a method of increasing resistance to carburization by performing surface treatment to form a layer with an increased concentration of Si or Cr.
However, in any of the above-described prior art methods, it is necessary to carry out special heat treatment or surface treatment, so these methods are uneconomical. In addition, these methods do not take into consideration restoration of scale (scale regeneration) when previously oxidized scale or a surface treatment layer peels off, so localized damage of scale is a problem.